PROJECT SUMMARY Changes to the extracellular matrix (ECM) help direct the inflammatory response to tissue injury and resolution of inflammation. During inflammation, TNF-stimulated gene-6 (TSG-6) covalently modifies the ECM polysaccharide hyaluronic acid (HA) with the heavy chains (HCs) from the serum protein inter-alpha-inhibitor (I?I). This cross-linking of HA is exclusively carried out by TSG-6 and only occurs during tissue injury, where inflammatory mediators are present to induce TSG-6 secretion and leaky vessels allow serum I?I to enter tissue. While HA cross-linking is present in histopathological lesions of various chronic lung diseases, the presence and role of TSG-6 mediated cross-linking in acute lung injury remains undefined. This proposal will establish for the first time the role of HA cross-linking in directing leukocyte recruitment and delaying resolution during acute lung inflammation. The clinically relevant murine model of Pseudomonas aeruginosa (PA) induced acute lung injury and repair will be used to test the hypothesis that TSG-6 mediated HA cross-linking mediates the recruitment of pro-inflammatory macrophages to the airway (Aim 1) and prolongs inflammation (Aim 2). In Aim 1, we will utilize mice deficient for TSG-6 and I?I as well as TSG-6 supplementation in I?I mice to specifically determine the role of TSG-6 mediated HA cross-linking in the recruitment of pro-inflammatory macrophages. In Aim 2, we will utilize TSG-6 loss of function approaches (TSG-6 null mice and neutralization antibody) to determine the effect of inhibiting TSG-6 on the rate of epithelial repair following PA infection. Completion of these aims will establish a new understanding of the role of TSG-6 induction in the lung and its evolutionarily conserved modifying of HA during inflammation that may yield new therapeutic strategies for lung repair.